In recent years, the social demands for weight saving of vehicles such as automobiles and the like have been increasing more and more due to consideration for global environment and the like. In order to respond to such requirement, as materials for automotive panels, in particular large body panels (outer panels and the inner panels) such as hoods, doors, and roofs, application of aluminum alloy materials having excellent formability and baking paint hardenability and lighter weight has been increasing, instead of steel materials such as steel sheets.
Among them, use of Al—Mg—Si-based AA or JIS 6000-series (hereinafter, also simply referred to as 6000-series) aluminum alloy sheet is studied as a thin-walled high strength aluminum alloy sheet for automobile panels of outer panels (outer sheets) and inner panels (inner sheets) of panel structures such as hoods, fenders, doors, roofs, and trunk lids.
The 6000-series aluminum alloy sheet essentially contains Si and Mg. Particularly, the excessive Si type 6000-series aluminum alloy has a composition in which Si/Mg is 1 or more in a mass ratio and has excellent aging hardenability. Therefore, the 6000-series aluminum alloy ensures formability at the time of press forming and bending processing based on low proof strength and has baking paint hardenability (hereinafter, also referred to as bake hardening properties=BH response and bake hardenability) that ensures required strength as panels because the proof strength is improved by aging hardening caused by heating at the time of artificial aging (hardening) treatment such as paint baking treatment of the panels after the forming.
The 6000-series aluminum alloy sheet has a relatively low alloy element amount compared with other aluminum alloys such as a 5000-series aluminum alloy having a large alloy amount such as a Mg amount. Therefore, when the scrap of the 6000-series aluminum alloy sheet is reused as an aluminum alloy melting material (melting raw material), the original 6000-series aluminum alloy ingots are easily obtained, and therefore the 6000-series aluminum alloy also has excellent recyclability.
However, even such a 6000-series aluminum alloy sheet has insufficient strength level after BH and thus further strength improvement is required for achieving lighter weight due to a thin wall thickness. In other words, when the 6000-series aluminum alloy sheets are used for pillars such as a center pillar, arms such as a side arm, or reinforcing members such as a bumper reinforcement and door beam, which are skeletal members or structural members, in a state of thin sheets, the strength after BH is insufficient. This problem also arises when the 6000-series aluminum alloy sheets are used for skeletal members or structural members other than the automotive use as the thin sheets.
Conventionally, various suggestions have been made for improving the BH response of the 6000-series aluminum alloy. For example, Patent Literature 1 suggests that strength change at room temperature after production be suppressed by changing a cooling rate stepwise at the time of the solution heat treatment and the quenching treatment to obtain the BH response. Patent Literature 2 suggests that the BH response and a shape fixability be obtained by, holding the aluminum alloy at a temperature of 50° C. to 150° C. for 10 minutes to 300 minutes within 60 minutes after the solution heat treatment and the quenching treatment. Patent Literature 3 suggests that the BH response and the shape fixability be obtained by regulating the cooling temperature at the first step and the cooling rate thereafter at the time of the solution heat treatment and the quenching treatment.
Patent Literature 4 suggests that the BH response be improved by heat treatment after solution hardening. Patent Literature 5 suggests that the BH response be improved in accordance with regulation by an endothermic peak measured with a DSC (Differential scanning calorimetry) method. Similar to Patent Literature 5, Patent Literature 6 suggests that the BH response be improved in accordance with regulation by an exothermic peak measured with DSC. However, for the cluster (aggregate of atoms) that directly affects the BH response of the 6000-series aluminum alloy sheet, these Patent Literatures 1 to 6 merely indirectly analogize the behavior of the cluster.
On the other hand, in Patent Literature 7, the cluster (the aggregates of atoms) that directly affects the BH response of the 6000-series aluminum alloy sheet is tried to be directly measured and regulated. More specifically, among the clusters (aggregates of atoms) observed by analyzing the microstructures of the 6000-series aluminum alloy sheet with a transmission electron microscope having a magnification of one million, an average number density of the clusters having a circle-equivalent diameter in a range of 1 nm to 5 nm is regulated in a range of 4000 clusters/μm2 to 30000 clusters/μm2 to obtain the 6000-series aluminum alloy sheet having excellent BH response and suppressing the natural aging at room temperature.
In Patent Literature 8, it has been found out that the cluster to which Mg atoms and Si atoms have a specific relation is correlated with the BH response by directly measuring the cluster that is significantly affected with the BH response by 3DAP described below. In addition, it has been found out that high BH response can be achieved by increasing the number density of the aggregates of atoms that satisfy these conditions even when automobile body paint baking treatment is carried out after the natural aging at room temperature.